Interpretive Summary: Citrus canker is an important disease of grapefruit in Florida. The distribution of disease on fruit surfaces was investigated on six samples of 24 diseased grapefruit from groves in east Florida. For all six samples lesions were enumerated on the upper (peduncle end) and lower (flower scar) halves, and on four of the samples lesions were counted on each quarter from the top down. 70-82% of all lesions were found on the upper half of the grapefruit. Sequentially on the four quarters, 40-47% of all lesions were found on the upper quarter of the fruit, and 28-39%, 9-16% and 7-10% of lesions were found on the lower three quarters, respectively. The highest count was consistently on the upper portions of the fruit. Presumably the upper surfaces of the fruit are more prone to infection as they have greater exposure to splash born inoculum.

Technical Abstract:
Citrus canker, caused by the plant pathogenic bacterium Xanthomonas citri subsp. citri (Xcc) is an important disease of grapefruit in Florida. To establish disease distribution on fruit, six samples of 24 diseased grapefruit were collected from two groves in east Florida. A plane was sliced through the middle of the fruit such that the vertical dimension of the fruit (the diameter, with peduncle at the apex) was split into equal sections. The surface area of each fruit half (hemisphere) was assumed to be the same. For all six samples of 24 fruit each of the lesions was enumerated on the upper (peduncle end) and lower (flower scar) halves. On four of the samples the fruit was sliced along three planes and the number of lesions enumerated on each of the four slices. On the six samples, 70-82% of all lesions were found on the upper half of the grapefruit. Sequentially on the four quartered samples, 40-47% of all lesions were found on the upper quarter of the fruit, and 28-39%, 9-16% and 7-10% of lesions were found on the lower three quarters, respectively. GLM analysis showed significant differences in lesion counts from each section; the highest count consistently being on the upper portions of the fruit. A logistic model described the relationship between lesion count and vertical distance from the fruit apex. Presumably the upper surfaces of the fruit are more prone to infection as they have greater exposure to splash born inoculum.